Operating Segments and Related Disclosures - Geographic Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 3,323,034	$ 2,643,552	$ 2,320,021
DOT—Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues									2,990,700	2,432,100	2,134,400
DOT—Latin America and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues									$ 332,300	$ 211,500	$ 185,600